The present invention relates to a method of injection molding a plastic lens and, more particularly, to temperature control of an injection molding assembly for making a highly precise lens molded in a cavity.
An injection molding technology to mold a meniscus-shaped plastic spectacle lens is shown in Japanese Patent Publication No. Hei 5-30608. In this technology, a cavity for molding the lens is formed inside an injection molding assembly, the cavity containing a pair of cavity forming members for shaping a convex surface and a concave surface of the lens disposed vertically opposite with each other. The injection molding assembly is heated before filling a molten resin in the cavity and one cavity forming member is moved toward the other cavity forming member to pressurize the molten resin filled in the cavity. Subsequently, the injection molding assembly is cooled to cool and solidify the molten resin, and the molded lens is taken out (=eject).
It is shown in Japanese Patent Laid-open No. Hei 6-31785 that an injection molding assembly is heated by means of a heating fluid such as steam and cooled by means of a cooling fluid such as air, water. In addition, after a molten resin is filled in a cavity in the injection molding assembly of which the temperature is raised beyond flow halting temperature of the molten resin, the temperature of the injection molding assembly is lowered below a glass transition point for molding a lens by cooling and solidifying the molten resin.
A lens is a precise molded product which requires high molding precision. Especially in a meniscus lens used for a spectacle lens, it is important that a convex shape and a concave shape of a pair of cavity forming members for shaping a convex surface and a concave surface of the lens are precisely transferred to the lens. However, when a lens to be molded has a difference in thickness between a central portion and a peripheral portion thereof, a thickness of the central portion being larger less than that of the peripheral portion, the lens is easy to bend at the thin central potion portion. When such a disadvantage occurs, a high-precision lens to which a convex shape and a concave shape of cavity forming members are accurately transferred is not obtained.
For manufacturing a high-precision lens, it is important to prevent heat distortion or shrinkage deformation from occurring, which requires that the entire molten resin filled in the cavity is uniformly cooled. However, since the amount of the molten resin filled in the cavity corresponds to the volume of the lens and differs depending on a type of the lens, especially lens power, uniform cooling is difficult by controlling the temperature uniformly. Thus, temperature control of an injection molding assembly is desired for molding each lens highly precisely irrespective of the above difference.
An object of the present invention is to provide a plastic lens injection molding method to mold a high-precision lens by means of proper temperature control of an injection molding assembly.
A method of injection molding a plastic lens according to the present invention provides a cavity for molding the lens formed by a pair of cavity forming members disposed opposite with each other inside an injection molding assembly for shaping a convex surface and a corresponding concave surface of the lens. The injection molding assembly is heated before filling a molten resin in the cavity and pressurizing the molten resin. Thereafter, the injection molding assembly is cooled to cool and solidify the molten resin for molding the lens in the cavity, before ejecting from the cavity. In the aforementioned method of injection molding the plastic lens, the temperature of the cavity forming member for shaping the lens convex surface is lowered below the temperature of the cavity forming member for shaping the lens concave surface in ejecting the lens.
According to the above injection molding method, when the lens is ejected, the temperature of the lens convex surface is lower than that of the lens concave surface, that is, the lens convex surface is cooled and solidified earlier from the lens concave surface, which prevents the lens from bending at a central portion thereof. Consequently, a high-precision lens can be obtained, where shapes of the convex surface and the corresponding concave surface of a pair of the cavity forming members are precisely transferred.
The aforementioned injection molding method is used for molding a meniscus lens, especially more effective in molding a lens having larger thickness of a peripheral portion than the thickness of a central portion (a minus lens).
When the minus lens is molded, it is preferable that the difference in temperature between the cavity forming member for shaping the lens convex surface and the cavity forming member for shaping the lens concave surface is enlarged in proportion to increase in the power (meaning spherical vertex refractive power and/or cylindrical refractive power in the present invention) of the lens molded in the cavity. Usually, as the lens power increases, the thickness of a peripheral portion becomes larger than that of a central portion, that is, a difference in thickness is enlarged, which causes the lens to bend easily at the central portion. However, the central portion of the lens is prevented from being bent even in a minus lens having large thickness difference by enlarging the temperature difference between the cavity forming member for shaping the lens convex portion and cavity forming member for shaping the lens concave portion in proportion to the increase in the lens power.
Time to cool the injection molding assembly after pressurization of the molten resin is preferably lengthened in proportion to the increase in the lens power in order to mold each of highly precise lenses having different power. As the lens power increases, the volume of the lens, that is, the amount of the molten resin filled in cavity increases. Therefore, the whole molten resin in the cavity can be gradually cooled uniformly to a predetermined temperature by lengthening a cooling time in proportion to the increase of the lens power. Consequently, each of lenses with different powers can be molded highly precisely with little heat distortion, little shrinkage deformation and the like.
In order to lower the temperature of the cavity forming member for shaping the lens convex surface below the temperature of the cavity forming member for shaping the lens concave surface, but the temperatures of the two cavity forming members may be the same or almost the same over the majority of the cooling time after pressurization of the molten resin. However, in order to securely lower the temperature of the lens convex surface below the temperature of the lens concave surface in ejecting, it is preferable that the cooling time of the injection molding assembly is controlled while differentiating the temperature of the cavity forming member for shaping the lens concave portion and the cavity forming member for shaping the lens convex portion by controlling flow rate of the temperature controlling fluid circulating in the injection molding assembly for raising and lowering the temperature of the injection molding assembly, thereby lowering the temperature of the cavity forming member for shaping the lens convex surface below the temperature of the cavity forming member for shaping the lens concave surface.
In the above, a pair of the cavity forming members for shaping the convex surface and the concave surface of the lens may be opposed with each other vertically or horizontally. In other words, an injection molding machine in which the injection molding assembly is mounted can be vertically or horizontally structured.
The number of cavities provided in the injection molding assembly is optional. One or more than one cavity is available.
In the method of injection molding the plastic lens according to the present invention, the molten resin is filled in the cavity for molding the lens inside the heated injection molding assembly and pressurized. Subsequently, the injection molding assembly is cooled to cool and solidify the molten resin so as to mold the lens in the cavity before ejecting the lens from the cavity. The injection molding method is characterized in that the time to cool the injection molding assembly is lengthened in proportion to increase in the power of the lens molded in the cavity.
According to the aforementioned injection molding method, since the cooling time of the injection molding assembly is controlled in proportion to change in power, the entire molten resin can be uniformly cooled to the predetermined temperature, thereby manufacturing a high-precision lens with little heat distortion, little shrinkage deformation and the like.
The injection molding method is available for molding, a lens of which a thickness of a peripheral portion is smaller than that of a central portion (a plus lens), and a semi-finished lens as well as the aforementioned minus lens.
Moreover, the injection molding method is applicable not only for molding a meniscus lens but for molding other types of lenses.
When the lens is a spectacle lens, some spectacle lenses have the same lens power, and different astigmatic powers. In such spectacle lenses with the same power and different astigmatic powers, it is preferable that the cooling time of the injection molding assembly is lengthened in proportion to increase in the astigmatic power, since the amount and/or the shape of molten resin filled in the cavity change in accordance with the astigmatic power even when the surface area of cavities in the injection molding assembly have no substantial difference.
The injection molding assembly can be heated and cooled by means of an electric heater, air cooling and the like. However, if the injection molding assembly is heated and cooled by means of a temperature controlling fluid of which the temperature is controlled, more specifically a heating fluid and a cooling fluid, temperature control can be performed highly precisely and easily.
When the lens is a spectacle lens, it is preferable that the temperature of the injection molding assembly is controlled in accordance with temperature curve, at least two temperature curves being prepared for weak power and strong power for minus lens and at least one for the plus lens. In molding a semi-finished lens of which one surface is later processed, it is preferable that another temperature curve is further prepared and the temperature of the injection molding assembly is controlled in accordance with the temperature curve. Thus, at least three temperature curves of the injection molding assembly, that is, for a weak minus lens, a strong minus lens and a plus lens, or four temperature curves can be prepared when a temperature curve for a semi-finished lens is included, so that temperature control can be easily conducted.
When the minus lens and the plus lens are molded, it is preferable that time to cool the injection molding assembly after pressurization of the molten resin is set for respective groups divided by lens spherical power and lens astigmatic power. Especially in molding the minus lens, the cooling time is preferably set for respective groups divided on the basis of the sum of lens spherical power and lens astigmatic power.